A CMOS Q-Enhancement Bandpass-Filter For Use In Paging Receivers

Paging receivers often have to work in a dense signal environment. This poses high demands on the preselection filter. One of the most difficult aspects is the large image rejection demand, which only can be satisfied by use of a narrow-band or high-Q filter. The practical restrictions for possible filter implementations are the low cost, low power and the small size of the pager. By use of positive feedback around a cheap off-chip low-Q inductor we obtain an enhanced quality factor. We are therefore able to construct selective filters using cheap small-size inductors. The price paid for Q-enhancement is a larger noise and higher sensitivity to component variations. The higher noise influence is eliminated using a high gain in the preceding LNA-stage, which is considered a part of the filter. Simulated results are: Q enhanced from 30 to 100, Image-rejection = 48dB, f0 = 280MHz, Voltage-gain = 20dB, Noise- figure = 2.4dB, IMFDR = 66dB, IDD = 1mA, VDD = 2V. The original contribution of this work is the application of the enhancement principle to off-chip components, which benefits the minimization of size and cost.